1. Field of the Invention
The present invention relates to methods and inorganic compositions, such as polyvalent metal silicates and polyvalent metal carbonates, in combination with copolymers of maleic anhydride and isobutylene for inhibiting the formation, deposition, and/or adherence of scale deposits on substrate surfaces in contact with a scale-forming aqueous system. The scale deposits may be alkaline earth metal scale deposits, such as alkaline earth metal carbonate scale deposits, especially calcium carbonate scale deposits, or alkaline earth metal oxalate deposits. The present invention may be advantageously used to prevent scale in a variety of processes such as kraft pulping processes.
2. Discussion of Background
Scale build-up is a serious problem in many industrial water systems, such as cooling towers, heat exchangers, evaporators, pulping digesters, washers, and in the production and processing of crude oil-water mixtures, etc. The build-up of scale deposits reduces the efficiency of heat transfer systems, interferes with fluid flow, facilitates corrosive processes and harbors bacteria. Calcium carbonate, generated in various processes, is one of the most commonly observed scale formers in industrial water systems. This scale is an expensive problem in many industries, which causes delays and shutdowns for cleaning and removal.
In particular, most industrial waters contain metal ions, such as calcium, barium, magnesium, aluminium, strontium, iron, etc. and several anions such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until product solubility concentrations are no longer exceeded. For example, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction products, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, change in pH, temperature or pressure, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on the surfaces of the water carrying system, they form scale or deposits.
For boiler systems and similar heat exchange systems, the mechanism of scale formation is apparently one of crystallization of scale-forming salts from a solution which is locally supersaturated in the region adjacent the heating surface of the system. The thin viscous film of water in this region tends to become more concentrated than the remainder of the solution outside this region. As a result, the solubility of the scale-forming salt reaction product is first exceeded in this thin film, and crystallization of scale results directly on the heating surface. In addition to this, a common source of scale in boiler systems is the breakdown of calcium bicarbonate to form calcium carbonate water and carbon dioxide under the influence of heat.
No For open recirculating cooling water systems, in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat by evaporation of water, the chief factor which promotes scale formation is concentration of solids dissolved in the water by repeated evaporation of portions of the water phase. Thus, even a water which is not scale forming on a once-through basis usually will become scale forming when concentrated a multiple number of times.
Also as disclosed in U.S. Pat. No. 3,518,204 to HANSEN et al., the disclosure of which is herein incorporated by reference in its entirety, water supplies employed as cooling media frequently contain silts such as bentonitic or kaolinitic minerals. During use of such silt containing waters in these systems, the silts react or associate with other impurities which are present in the water such as calcium and magnesium which are commonly referred to as "hardness". As a consequence of such reaction or association, a precipitate is formed and precipitated upon the surfaces of the system containing the water. Such depositions may build up to the extent that flow through the system is reduced or halted, and the system must be shut down for costly cleaning. In addition, when such deposition occurs on heat transfer surfaces, heat exchange is reduced with a corresponding loss in process efficiency.
Scaling in kraft pulping processes occurs by a different mechanism as a result of the presence of organic ligands. Black liquor generated in the kraft pulping digester contains a very high content of organics such as lignin, fatty/rosin soaps, hemicelluloses, etc. Lignin fragments formed during pulping, specifically those containing adjacent hydroxyl groups on an aromatic ring, have a high tendency to interact with calcium (originally from tree) to greatly increase its solubility in black liquor. As the temperature increases (e.g., the temperature found near the tube wall of an evaporator or cooking heater), the pH has a tendency to decrease, especially if the residual active alkali is low. As a consequence, calcium ions can be displaced from the lignin by hydrogen ions, and react with carbonate ions thus producing calcium carbonate scale. In addition to lignin, there are many different organic species that complex calcium in the black liquor. Any of these organic species, whose ability to complex with calcium depends on the pH being in the normal pH range of black liquor, will contribute to calcium carbonate scaling by releasing ionic calcium as the temperature increases. Therefore, as long as some of the aforementioned organic compounds are present and sufficient calcium is available, a liquor will have the capacity to deposit calcium carbonate scale. In addition to calcium and carbonate, black liquor normally contains a number of other ions such as sodium and sulfate which can precipitate and form scale.
In the paper industry, alkalinity from alkali digesting solution and from dissolved solids from the wood chips, results in an increased alkalinity of the black liquor, often reaching pH's of 12-13 and even higher. Under high pH conditions, the precipitation of calcium carbonate is especially difficult to control. Acid is often added to lower the pH to prevent calcium carbonate scaling.
In the papermaking process, calcium oxalate scale often forms on process equipment during the bleaching/delignification of pulp by chlorine, caustic soda, chlorine dioxide, hypochlorite and peroxide. Usual areas of scale build-up are on washer drum face wires; in washer vats; in stock lines and pumps; in filtrate tanks, lines, and pumps; on extraction screens; and in treatment towers. The formation of calcium oxalate scale provides an economic hardship on mills principally because of lost production due to decreased bleaching/delignification efficiency and equipment downtime associated with the removal of scale.
In the oil industry, the formation of insoluble calcium salts is also a problem in the secondary recovery of oil from subterranean formations by processes in which water is introduced into injection wells and forced through the underground formations to cause oil to be produced in a producing well. This type of process is usually referred to as a waterflood system.
In view of the above, scale formation and deposition are generated by the mechanisms of nucleation, crystal growth, and aggregation of scale-forming particles. Various approaches to reducing scale development include inhibition of nuclei/crystal formation, modification of crystal growth, and dispersion of the scale-forming particles.
Chelating or sequestering agents have been commonly used to prevent deposition, precipitation and crystallization of calcium carbonate in water-carrying systems. Other types of chemicals which have been actively explored as calcium carbonate scale inhibiting agents are threshold inhibitors.
Threshold inhibitors include water soluble polymers, phosphonates, and polyphosphates (e.g., U.S. Pat. No. 5,182,028 to BOFFARDI et al., the disclosure of which is herein incorporated by reference in its entirety, discloses sodium hexametaphosphate and monofluorophosphate). Such chemicals are effective as scale inhibitors in amounts considerably less than that stoichiometrically required.
Water soluble polymers, including groups derived from acrylamide, maleic acid, vinyl acetate, vinyl alcohol, and acrylic acid have been used to control calcium carbonate deposition. For instance, such polymers are disclosed in U.S. Pat. No. 5,282,976 to YEUNG; U.S. Pat. No. 5,496,914 to WOOD et al.; U.S. Pat. No. 4,008,164 to WATSON et al.; U.S. Pat. No. 3,518,204 to HANSEN et al.; U.S. Pat. Nos. 3,928,196 and 4,936,987 to PERSINSKI et al.; U.S. Pat. No. 3,965,027 to BOFFARDI et al.; U.S. Pat. No. 5,441,602 to HARRIS et al.; U.S. Pat. No. 5,580,462 to GILL; and U.S. Pat. No. 5,409,571 to TOGO et al., the disclosures of which are herein incorporated by reference in their entireties.
Polyallylamines having phosphonic, carboxylic, or sulfonic groups are also used as scale control agents as disclosed in U.S. Pat. No. 5,629,385 to KUO and U.S. Pat. No. 5,124,046 to SHERWOOD et al., the disclosures of which are herein incorporated by reference in their entireties.
Additionally, a number of anionic polyelectrolytes, such as polyacrylates, polymaleic anhydrides, copolymers of acrylates and sulfonates, and polymers of sulfonate styrenes, have been employed. Examples of polyelectrolytes are disclosed in U.S. Pat. No. 4,640,793 to PERSINSKI et al.; U.S. Pat. No. 4,650,591 to BOOTHE et al.; U.S. Pat. No. 4,457,847 to LORENC et al.; U.S. Pat. No. 5,407,583 to GILL et al.; and U.S. Pat. No. 4,671,888 to YORKE, the disclosures of which are herein incorporated by reference in their entireties.
Polyepoxysuccinic acid for inhibiting the formation and deposition of scale in aqueous systems is disclosed in U.S. Pat. Nos. 5,062,962 and 5,147,555 to BROWN et al., the disclosures of which are herein incorporated by reference in their entireties.
Phosphonate based compounds are extensively used as calcium carbonate scale control agents. Examples include ether diphosphonate (U.S. Pat. No. 5,772,893 to REED et al., and U.S. Pat. No.5,647,995 to KNELLER et al., the disclosures of which are herein incorporated by reference in their entireties), hydroxyethylidene-1,1-diphosphonic acid, amino tri(methylene phosphonic acid), aminomethylene phosphonates (U.S. Pat. No. 4,931,189 to DHAWAN et al., the disclosure of which is herein incorporated by reference in its entirety), N,N-bis(phosphonomethyl)-2-amino-1-propanol (U.S. Pat. No. 5,259,974 to CHEN et al., the disclosure of which is herein incorporated by reference in its entirety), methylene phosphonates of amino-terminated oxyalkylates (U.S. Pat. No. 4,080,375 to QUINLAN, the disclosure of which is herein incorporated by reference in its entirety), polyether polyamino methylene phosphonates (EP 0 516 382 B1, the disclosure of which is herein incorporated by reference in its entirety), and ethanolamine N,N-dimethylene phosphonic acid (U.S. Pat. Nos. 2,917,528 and 2,964,549 to RAMSEY et al., the disclosures of which are herein incorporated by reference in their entireties).
Additionally, it is known that certain inorganic polyphosphonates would prevent precipitation when added in amounts less than the concentrations needed for sequestering or chelating, as disclosed in U.S. Pat. No. 2,358,222 to FINK et al. and U.S. Pat. No. 2,539,305 to HATCH, the disclosures of which are herein incorporated by reference in their entireties.
U.S. Pat. No. 3,960,576 to CARTER et al., the disclosure of which is herein incorporated by reference in its entirety, discloses that inorganic-silicate-based compositions also comprised of an organic phosphonate and carboxy methyl cellulose are useful for inhibiting corrosion of metal surfaces.
MANAHAN, Environmental Chemistry, pp. 183-213 (1991), the disclosure of which is herein incorporated by reference in its entirety, with particular attention directed to pp. 193-195, discloses use in environmental chemistry of sodium aluminum silicate minerals or zeolites as water softeners. The softening of water by aluminum silicate minerals and zeolites is based on ion-exchanging properties of the minerals. The divalent cations, which are responsible for water hardness, are replaced by sodium ions contained in the aluminum silicates, and then removed by filtration. An example of a micaceous mineral which has been used commercially in water softening is glauconite, K.sub.2 (MgFe).sub.2 Al.sub.6 (Si.sub.4 O.sub.10).sub.3 OH.sub.12.
Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., vol. 24, pp. 367-384 (1984), the disclosure of which is herein incorporated by reference in its entirety, discloses that deposits are usually controlled with dispersants and scale inhibitors in cooling and process water. Among the dispersants mentioned are polymers and copolymers, for example, poly(acrylic acid) and its salts, acrylamideacrylic acid copolymers and poly(maleic acid).
"Deactivation of Calcium Scaling Liquors", The Members of the Paper Institute of Paper Chemistry, Project 3234, Report Three, pp. 88-119 (November 1977), the disclosure of which is herein incorporated by reference in its entirety, discloses adding reagent grade calcium carbonate at 1% loading in most experiments and at 5% and 20% in a few other experiments, to function as a seed in the liquor as a deposition surface for calcium carbonate.
ADAMS, "Low-Cost Evaporator Upgrades Boost Performance, Reduce Scaling", Pulp & Paper, pp. 83-89 (February 1999), discloses a salting method which involves adding sodium sulfate to control scaling.
CA 2,229,973 discloses a method of inhibiting black liquor in evaporators, wherein the liquor is heat-treated to precipitate calcium carbonate. This document discloses that no calcium carbonate needs to be added to the liquor to be heat-treated.
EP 0 916 622 discloses a process for preventing scale formation in a paper-making process, wherein calcium sulfate or calcium oxalate are added as a seed to prevent formation of calcium sulfate scale or calcium oxalate scale, respectively.
Further, it is known to use clays such as talc and bentonite in paper making for fillers, pitch control, and retention and drainage control. In filler applications, talc or bentonite may be added in an amount which is typically relatively high.
In pitch control applications, talc or bentonite may be added before the washer and after the digester. At this position, the temperature of the aqueous system is relatively low. The use of talc and bentonite for pitch control is discussed in BOARDMAN, "The Use of Organophilic Mineral Particulates in the Control of Anionic Trash Like Pitch", TAPPI Proceedings (1996), the disclosure of which is herein incorporated by reference in its entirety. In particular, this article discloses using two pounds per ton of montmorillonite. It is known that pitch deposits may sometimes include calcium carbonate.
In retention and drainage control, it is believed that bentonite and a high molecular weight cationic polymer (e.g., molecular weight of about 1.times.10.sup.6 to 10.times.10.sup.6) may be added just before the headbox. For instance, it is believed that 3-10 lb of bentonite/ton of oven dried fibers may be added near the headbox which would result in about 15-50 ppm of bentonite in the aqueous system for a 1 wt % aqueous paper furnish. It is believed that the aqueous system just before the headbox typically has a pH of about 5 to 8.5 and a temperature of about 40.degree. C. to 60.degree. C. As an example, U.S. Pat. No. 4,753,710 to LANGLEY et al. teaches that the bentonite particle size after swelling is preferably at least 90% below 2 microns.